A shadow mask is provided with many apertures through which electron beams emitted from an electron gun pass. The electron beams passing therethrough cause phosphors to be deposited on the inner surface of a face plate for emitting light. However, at the same time, a part of electron beams impinge the surface formed between the apertures, so that a dome is formed in the shadow mask by the heat generated from the impinging beams.
AS a result, the apertures of the shadow mask discontinue a scanning path thereby causing the deterioration of picture quality. Conventionally, to overcome this problem, when installing a frame at the inside of the face plate, a hook spring is placed to offset deformation of shadow mask due to the doming. However, this does not provide a structural solution for preventing or removing the doming thereof.
Another method to solve the doming problem is to provide a shadow mask made of invar alloy having extremely low thermal expansion coefficient. However, invar alloy is difficult to form and the cost thereof is high.
U.S. Pat. No. 4,442,376 describes a method involving coating a heavy metal layer on the side of the shadow mask facing the electron gun so that the electron beams impinging the shadow mask is reflected thereby preventing thermal expansion thereof. However, expansion of a shadow mask is also caused by the radiant heat at the inside of a color cathode ray tube so this is not a complete solution.
In an attempt to avoid the problems described above, recently a method of coating an anti-doming material on the surface of the shadow mask has been studied.
The anti-doming material used in this method expends backward in order to offset the deformation due to the doming of the shadow mask, so that the deformation by the doming is prevented. As materials effective in the anti-doming material, the possibilities aluminum having high thermal expansion coefficient and boron nitride ceramic having low thermal expansion coefficient. Aluminum used as the anti-doming material is easily oxidized when exposed to the air, so that as a antidoming material the boron nitride ceramic is more stable than aluminum. However, the boiling point of the boron nitride ceramic is more than 3000.degree. C., so that when depositing the boron nitride ceramic, extraordinary deposition equipment which can endure the high temperature must be provided and the deposition process is complicated so there is no advantage.